try to preload certain applets in a smart way

Summary:
preload popups of some applets after init in the background
based on a value of X-Plasma-PreloadWeight in the desktop file
if present, otherwise some default values based on the applet
type (Provides)

Save the weight in the config, if an applet is never opened,
slowly decrease the weight, when it reaches 0 don't preload it
next start, increase every time it gets opened, so at the moment
it's quite aggressive about preloading, in order to not do it
a lot of plasma startups without touching the applet are needed

Applet with a very big weigth will be preloaded immediately,
therefore having an impact on the time it will take to have
a panel visible and usable, while lesser weigths will preload
after a random number of seconds between 2 and 10, so will load
in the background after everything is started

Test Plan:
Plasma starts up correctly, applets load correctly and can be added
correctly both those expanded or collapsed.
plasmashell appears correctly usable without too big hiccups even
while it's loading things in the background

some numbers:
without preloading, plasma takes around 64 mb of memory after startup
when preloading everything about 94, so it's a cost of about 30 mb
which is not negligible.
don't have precise timing, but if everything gets preloaded immediately,
the time to get an usable desktop appears to be at least doubled,
while the delayed preloading (except just a couple of applets) doesn't
seem to have a big impact on the time needed to get an usable desktop

Reviewers: Plasma

Subscribers: davidedmundson, broulik, apol, ngraham, plasma-devel, Frameworks

Tags: Plasma, Frameworks

Differential Revision: https://phabricator.kde.org/D10512